Certain contaminants in monitor tube phosphors, believed to be heavy metals; i.e., metals having a specific gravity greater than 5.0, cause a majority of the scrap items in the production of these tubes. The problem manifests itself as numerous black spots appearing on the screen; these spots being visible mainly after bake-out. Previous attempts to solve this problem and rid the screen of these deleterious black spots have involved double baking the screens and keeping the phosphor suspension fresh. Results from these techniques have proven ineffectual, occasionally performing their function but often showing no improvement. Furthermore, application of these techniques induce production bottlenecks which increase the cost in a highly competitive industry and are thus economically unfeasible.